The present invention is generally directed to visual rehabilitation and treatment of pain for patients with epithelial defects on the cornea of the eye. Although specific reference is made to epithelial defects following photorefractive keratectomy, embodiments of the present invention can be used to treat epithelial defects from other causes, for example corneal abrasions, trauma, keratoconus, penetrating keratoplasty and dystrophies.
The eye includes several tissues that allow patients to see. The cornea of the eye is an anterior tissue of the eye that is clear in healthy eyes and refracts light so as to form an image on the retina. The retina is a posterior tissue of the eye that senses light from the image formed thereon and transmits signals from the image to the brain. The cornea includes an outer layer of tissue, the epithelium, which protects the underlying tissues of the cornea, such as Bowman's membrane, the stroma and nerve fibers that extend into the stroma and Bowman's. The healthy eye includes a tear film disposed over the epithelium. The tear film can smooth small irregularities of the epithelium so as to provide an optically smooth surface. The tear film is shaped substantially by the shape of the underlying epithelium, stroma, and Bowman's membrane, if present. The tear film comprises a liquid that is mostly water and does include additional components, such as mucoids and lipids. The many nerve fibers of the cornea provide sensation to promote blinking that can cover the cornea with the tear film. The never fibers also sense pain so that one will normally avoid trauma to the cornea and also avoid direct contact of an object to the cornea so as to protect this important tissue.
In the healthy cornea, the proper amount of hydration of the cornea, sometimes referred to as dehydration of the cornea, is maintained such that the cornea remains clear. The cornea includes a posterior endothelial layer that pumps water from the cornea into the adjacent anterior chamber. The epithelium minimizes flow of water from the tear liquid into the cornea, such that the corneal stroma can be maintained with the proper amount of hydration with endothelial pumping. The endothelial pumping of water from the cornea to maintain the proper hydration and thickness of the eye is often referred to as deturgescence.
In patients with epithelial defects, the barrier function of the epithelium is compromised, such that water can enter the cornea through the epithelial defect so as to cause swelling of the corneal stroma. As a result, excessive hydration of the cornea may occur in eyes with epithelial defects. In some instances, excessive hydration that swells the corneal stroma can result in light scattering, or haze, such that an image seen by a patient is degraded. The scattering of light by the corneal stroma can be seen with a slit lamp examination to diagnose the patient, and is sometimes referred to as corneal haze. In addition to potentially causing excess hydration of the cornea, an epithelial defect can expose the nerve fibers of the cornea such that the patient feels pain.
Several known techniques exist to treat corneal epithelial defects, including bandage therapeutic lenses, non-steroidal anti-inflammatories (hereinafter NSAIDS), steroids, antibiotics and analgesics. These known techniques may be somewhat effective in reducing symptoms associated with the epithelial defect. However, many of these known techniques may not provide a barrier to water entry into the corneal stroma, such that the cornea may swell with water and may affect patient vision in at least some instances. For example, a bandage therapeutic lens may be placed over the epithelial defect to cover and protect the corneal tissues under the defect, such as the corneal stroma and nerve fibers. However, in at least some instances the bandage therapeutic lens may not prevent water of the tear from leaking through the epithelial defect into the stroma. Also, a bandage therapeutic lens may slide over the epithelial defect when positioned on the eye in at least some instances, potentially decreasing the therapeutic benefit of the therapeutic lens when the lens slides along the delicate underlying tissue, for example when a patient blinks.
Work in relation to embodiments of the present invention suggests that at least some of the known therapeutic bandage lenses used to treat epithelial defects may actually contribute to corneal edema and pain in at least some instances. At least some of the current bandage lenses may provide less oxygen than would be ideal, and decreased oxygen to the cornea may be related pain and corneal edema in at least some instances. Also, in at least some instances, bandage lenses may be fit loosely on the cornea, such that water can go around the bandage lens and may penetrate the stroma through the epithelial defect.
Although analgesics such as lidocaine may reduce pain, the overuse of these treatments can delay regeneration of the epithelial tissue over the defect, such that the defect may last longer. Consequently many people with epithelial defects may feel pain and have degraded vision while the epithelial defect heals.
Many people elect to undergo laser vision correction surgery to treat refractive error of the eye, such as near sightedness. With one form of this surgery known as photorefractive keratectomy (hereinafter “PRK”), a large area of the epithelium is removed, for example a 6 mm area. Following ablation of the underlying tissues such as the corneal stroma and/or Bowman's membrane, the epithelium grows back over the ablation to cover the area where the epithelium was removed. This re-growth of the epithelium can take three to four days, and at least some of the patients who undergo this surgery may feel pain. In addition, the epithelium may be somewhat irregular while growing back over the corneal stroma, and the irregularities may degrade patient vision in at least some instances. Further, work in relation to embodiments of the present invention suggests that anterior stromal edema, ablated surface irregularities and necrotic cells in the ablated surface area may decrease vision in some instances. Therefore, improved treatment of epithelial defects may result in improved patient comfort and vision following PRK, and possibly other surgeries that remove the corneal epithelium.
In light of the above, it would be desirable to provide improved treatments for epithelial defects of the cornea. Ideally, these treatments would avoid at least some of the deficiencies of known techniques while providing improved patient comfort and/or vision while the epithelial defect heals.